(a) Field of the Invention
This invention relates to a cardiac pacemaker suitable for implantation in a human or animal body as a part of a complete cardiac pacemaker system. A complete cardiac pacemaker system may additionally comprise a plug and socket connector (such as is described and claimed in my co-pending application Ser. No. 884,830 filed on the same day as this application in my name alone and entitled "Plug and Socket Connectors," the entire disclosure of which is incorporated herein by reference) and an electrode catheter to carry electric pulses to the heart (such as is described and claimed in my co-pending application Ser. No. 884,967, now abandoned, filed on the same day as this application in my name alone and entitled "Electrode Catheter," the entire disclosure of which is incorporated herein by reference).
(b) Description of the Prior Art
In a cardiac pacemaker system, the implanted pacemaker itself, which supplies pulses at regular intervals to stimulate the heart via an electrode catheter, must have a long and reliable life because surgery is required to correct any fault and of course in an extreme case the consequence of a pacemaker failure may be fatal. It is therefore most important that the case within which the power source and electronic components for generating the train of electric pulses are housed must be totally hermetically sealed and resistant to the body fluids present at the site of implantation. Cases have been made of metals such as stainless steel and titanium, but in the environment within a body, these metals suffer from electrolytic corrosion, especially if used as an electrode for return electric current. This corrosion may effect the bio-compatibility after a period of time. Plastics materials, such as epoxy resins, have been employed for implantation pacemaker cases, but moisture from the body can cause the resins to swell, leading eventually to cracking. Plastics cases thus have a typical life of 3 years. Polypropylene is another plastics material currently often employed for implant cases, but moisture tends to diffuse through polypropylene over a period of time and this can lead to premature failure of the contained electronic components. Also, when using a plastics material, it is necessary to provide a contact on the outer surface of the case to form an "earth return" for the electric circuit and this also can cause problems. In the first place, electrolytic corrosion may take place, but also if the plastics material is liable to change its physical structure or dimensions over a period of years, the security of attachment of the contact pad and the hermetic seal of a wire leading thereto through the case may fail.
In view of all of the above difficulties, there has been no commercially successful implant case which has been entirely free of problems for the life of the contained components. It has been appreciated that because platinum is relatively inert in a human or animal body and is bio-compatible, it would be a suitable material for a case, but in view of the cost of platinum it is not practical to make a case thereof. Platinum with a sufficient degree of purity to ensure bio-compatibility is relatively soft and this means that a case made of platinum must have a considerable wall thickness, thus putting up the cost yet more. Moreover, the weight of a sufficiently strong case can cause difficulties when the case has been implanted.